score:11
If your connecting flight is cancelled before you depart for NRT, the airline will update your itinerary, and you have the right to insist on a new itinerary, potentially through a different connecting airport, that is valid in terms of visas, COVID entry requirements etc. (That said, if they can't find one, the airline also has the right to cancel your itinerary entirely and refund you the money.)
If your connecting flight is cancelled after you're already on your way, the same will happen, only there will be less options because you're already at NRT and they may not be able to find an option that departs on the same day.
If you need to stay overnight, the airline will liaise with Japanese authorities. In normal times, French citizens could simply enter Japan for up to 90 days; for other nationalities, they would likely grant a Shore Pass valid up to 72 hours that will let you leave the airport freely. However, thanks to COVID, you're more likely to end up in a quarantine facility of some sort (which may well be a hotel near the airport). Nevertheless, you're not going to be detained or sent back, and the airline should cover the cost of accommodation and meals because this was their fault.